Ultra-Wideband (UWB) is a technology for transmitting information spread over a large bandwidth (>500 MHz) that can share spectrum with other users. Current Federal Communications Commission (FCC) rules are intended to provide an efficient use of radio bandwidth while enabling both high data rate personal area network (PAN) wireless connectivity and longer-range, low data rate applications as well as radar and imaging systems.
Although Ultra Wideband was originally pulse radio, the FCC and ITU-R currently define UWB in terms of a transmission from an antenna for which the emitted signal bandwidth exceeds the lesser of 500 MHz or 20% of the center frequency. Thus, pulse-based systems, wherein each transmitted pulse instantaneously occupies the UWB bandwidth, or an aggregation of at least 500 MHz of narrow band carriers, such as in orthogonal frequency division multiplexing (OFDM), can access the UWB spectrum. Each pulse in a pulse-based UWB system occupies the entire UWB bandwidth, thus reaping the benefits of relative immunity to multipath fading (but not to intersymbol interference), unlike narrowband carrier-based systems that are subject to both deep fades and intersymbol interference. In the United States, the FCC has mandated that UWB radio transmission may operate in the range from 3.1 GHz to 10.6 GHz, at a transmit power of −41 dBm/MHz.
Ultrawideband (UWB) is a wireless communications technology that operates in unlicensed spectrum. Advantages of UWB include low power consumption, very low cost/complexity, high data rates (up to 480 Mbps) and throughput and precision location capability. The specifications for UWB target emerging wireless personal area network (WPAN) communications. WPAN technology enables high-speed, short-range, cable-free connectivity for a wide array of multimedia consumer electronics, PC peripherals and mobile devices, including wireless USB and wireless 1394.
For example, several intended uses for UWB include sharing photos, music, video, data and voice among networked consumer electronics, PCs and mobile devices throughout the home and also remotely. For example, using UWB links, users will be able to stream video content from a PC or consumer electronics (CE) device such as a camcorder, DVD player or personal video recorder (PVR) to a flat screen high-definition television (HDTV) display without the use of any wires.
The digital home includes high-speed data transfer for multimedia content, short-range connectivity for transfer to other devices, low power consumption due to limited battery capacity and low complexity and cost. Example applications include transfer of video from a camcorder to an entertainment PC, the ability to view photos from the user's digital still camera on a larger display, removing all wires to and between printers, scanners, mass storage devices and portable CE audio/video (A/V) devices.
A traditional UWB transmitter works by sending billions of pulses across a very wide spectrum of frequency several GHz in bandwidth. The receiver then translates the pulses into data by listening for a familiar pulse sequence sent by the transmitter. Modern UWB systems use other modulation techniques, such as Orthogonal Frequency Division Multiplexing (OFDM), to occupy these extremely wide bandwidths. In addition, the use of multiple bands in combination with OFDM modulation can provide significant advantages to traditional UWB systems. The MultiBand OFDM approach allows for good coexistence with narrowband systems such as 802.11a, adaptation to different regulatory environments, future scalability and backward compatibility. This allows the technology to comply with local regulations by dynamically turning off subbands and individual OFDM tones to comply with local rules of operation on allocated spectrum.
With the formation of the MultiBand OFDM Alliance (MBOA) in June 2003, OFDM for each subband was added to the initial multiband approach in order to develop the best technical solution for UWB. To date, the Multiband OFDM Alliance has more than 170 member that support a single technical proposal for UWB. The MBOA is tasked with delivering the best overall solution for UWB with maximum emphasis on peaceful coexistence with other wireless services and to provide the most benefits to the broadest number of consumers end users. In the MultiBand OFDM approach, the available spectrum of 7.5 GHz is divided into several 528-MHz bands. This allows the selective implementation of bands at certain frequency ranges while leaving other parts of the spectrum unused. The dynamic ability of the radio to operate in certain areas of the spectrum is useful because it can adapt to regulatory constraints imposed by governments around the world.
The band plan for the MBOA proposal has five logical channels. Channel 1, which contains the first three bands, is mandatory for all UWB devices and radios. Multiple groups of bands enable multiple modes of operation for MultiBand OFDM devices. In the current MultiBand OFDM Alliance proposal, bands 1 to 3 are used for Mode 1 devices (mandatory mode), while the other remaining channels (2 to 5) are optional. There are up to four time-frequency codes per channel, thus allowing for a total of 20 piconets with the current MBOA proposal. In addition, the proposal also allows flexibility to avoid channel 2 when and if Unlicensed-National Information Infrastructure (U-NII) interference, such as from 802.11a, is present.
The information transmitted on each band is modulated using OFDM. OFDM distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the orthogonality in this technique, which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high-spectral efficiency, resiliency to RF interference and lower multipath distortion.
OFDM is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data are divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not required. OFDM, however, requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference (ICI), i.e. cross-talk between the sub-carriers. Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement. In most OFDM radio designs, a zero-IF scheme is used to convert the RF signal into a baseband signal without use of an intermediate frequency (IF) in the receiver and to convert the baseband signal into an RF signal without an IF in the transmitter. Being an OFDM based system, UWB uses the well-known zero-IF architecture for both receive (RX) and transmit (TX). A zero-IF transmitter, however, has disadvantages in that it is impossible to generate I and Q signal that are perfectly balanced through the upconverting process using orthogonal modulation. These shortcomings are caused from inaccuracies of analog circuits such as 90 degree shifting of a local oscillator (LO) signal and mismatching of mixers and filters. The impossibility of perfect balance results in a mismatch between the I and Q signals transmitted. This results in a degradation of EVM due to the TX IQ mismatch.
Prior art IQ calibration techniques exists to combat the IQ mismatch problem. These prior art techniques, however, all use open loop correction techniques to extract the phase and magnitude using a power detector. Other prior art techniques operate on the receive side and not at the transmitter. Disadvantages of these systems include (1) sensitivity of the IQ calibration process to the LO leakage level; (2) sensitivity of the IQ calibration process to the noise present in the system; (3) the use of approximations to avoid complex calculations result in significantly reduced accuracies.